For the most part, pure argon or a gas mixture containing argon and helium, hydrogen, oxygen and carbon dioxide is used for TIG, MIG, MAG and plasma welding. This process calls for an inert gas with a high degree of purity because impurities such as, for example, moisture, can lead to serious welding flaws. Commercially available welding argon, for instance, has a degree of purity higher than 99.996 percent, that is to say, the sum of all impurities can amount to 40 ppm at the maximum. The same applies for mixtures of argon and helium. As long as the inert gas mixture does not contain any helium, the proportion of argon is above 50 percent, in most applications, it lies at 80 to 90 percent. In the case of mixtures containing helium, the proportion of argon usually amounts to 50 percent, and sometimes helium proportions above 50 percent are employed.
Considerations from the point of view of metallurgy and process technology are the decisive factors in the selection of the inert gas. Since this entails observing a number of partially conflicting criteria, this necessarily calls for compromise. Important criteria in the selection of the inert gas for TIG welding are, for example, ignition behavior, electrode strength, arc stability, fusion penetration profile, viscosity of the melt and metallurgical properties of the melt.